1. Field of the Invention
The present invention relates to a boot for a constant-velocity universal joint, including a large-diameter side end portion attached to an outer peripheral surface of a tripod joint whose outer peripheral surface is formed to be concave and in which a plurality of axial-direction grooves are formed among constant-velocity universal joints for use, for example, in driving shafts for transmitting powers to hubs to which tires are attached from differential gears connected to engines of cars.
2. Description of the Related Art
Constant-velocity universal joints are used, for example, in opposite end portions of a driving shaft for a car. Moreover, a flexible boot is attached to cover a bent portion of the constant-velocity universal joint in order to seal grease for lubricating the constant-velocity universal joint and to prevent invasion of foreign matters such as dust and water from the outside. A large-diameter side end portion and a small-diameter side end portion of the boot are usually fastened to the outer peripheral surface of a casing of the constant-velocity universal joint on a differential gear side or a hub side and the outer peripheral surface of a shaft portion of the driving shaft by bands to fix the boot.
Moreover, in the constant-velocity universal joint on the differential side (inboard side), a tripod joint is generally used in which, for example, three sets of rollers attached to the shaft portion of the driving shaft in a trifurcate state are constituted to be slidable in an axial direction. In the outer peripheral surface of the casing of the tripod joint, groove-like concave portions extending in the axial direction in the outer peripheral surface are formed, for example, in three places scattered in a peripheral direction for thinning and lightening of the joint. In this case, in the inner peripheral surface of the large-diameter side end portion of the boot for the constant-velocity universal joint, a thick portion is formed in a circularly protruded shape as viewed in the axial direction in accordance with the surface of the concave portion (see U.S. Pat. No. 5,529,538, for example).
In the boot for the constant-velocity universal joint, which has heretofore been used in the tripod joint, for example, it is known that bellows are formed of resin, for example, by injection blow molding, and an annular grommet including a thick portion protruding in an inner-diameter direction is fitted into the inner peripheral surface along which the inner diameter of the large-diameter side end portion of the resin bellows is formed to be constant (see U.S. Pat. No. 5,529,538, for example).
In the boot for the constant-velocity universal joint, the casing of the tripod joint is inserted into an inner periphery of the grommet, and an outer periphery of the casing of the tripod joint is fastened by a fastening tool such as a band from an outer peripheral side of the large-diameter side end portion to attach the boot.
It is also known that the resin grommet including thick portions formed to protrude at each certain interval in the inner-diameter direction is molded beforehand, the grommet is held in a mold, and thereafter the resin bellows are injection-molded or blow-molded to integrate the grommet and bellows in the mold (see Japanese Utility Model Application Laid-Open No. 2-22463 and Japanese Patent Application Laid-Open No. 2002-286048, for example).
However, when the bellows and the grommet are separately molded, and fitted/assembled into each other as in a constitution described in U.S. Pat. No. 5,529,538, and when the grommet is fitted into the inner periphery of the bellows large-diameter side end portion, the bellows and the grommet shift from each other unless a fitting operation is carefully performed. Since the shifting causes grease leakage, an assembly operation becomes intricate in order to prevent the shifting.
Moreover, after attaching the grommet to the outer periphery of the tripod joint, the large-diameter side end portion of the bellows may also be attached from the outside. Even in this case, unless the operation is carefully performed, the bellows large-diameter side end portion and the grommet shift from each other, and there is a possibility that the grease leaks.
Furthermore, in the prior art described in Japanese Utility Model Application Laid-Open No. 2-22463 and Japanese Patent Application Laid-Open No. 2002-286048, any special technical means is not applied to a place where the grommet and bellows are integrated. Therefore, even when they are apparently integrated, they are not firmly fusion-bonded to each other, the apparently integrated place peels, and, as a result, a disadvantage such as a grease leakage is caused.
To solve the problem, the present inventors have succeeded in development of a newly useful method in which a core mold is disposed in a large-diameter side end portion of a primary molded member, molded beforehand integrally with a bellows portion and a small-diameter side end portion in primary molding, and held in a mold. Moreover, a secondary molded member including a thick portion adapted to an axial-direction groove in the outer peripheral surface of a tripod joint casing is injection-molded between a large-diameter side end portion inner surface of the primary molded member and a core mold outer peripheral surface. Accordingly, disadvantages of the above-described prior arts, such as operation intricacy and grease leakage have been solved (Japanese Patent Application No. 2003-041317).
Moreover, the present inventors have further found a problem in the newly developed inventive useful technique.
That is, in the manufacturing method developed by the present inventors, in consideration of ease of extracting of the boot from the core mold after the secondary molding, the thick portion has been protruded as little as possible in a boot axial center direction from the inner surface of a small-diameter portion just in the vicinity of the large-diameter side end portion in the bellows portion. However, when the end portion of the thick portion is positioned in the vicinity of the bellows portion on the inner surface of the small-diameter portion, flexing properties of the small-diameter portion (referred to also as the valley portion) just in the vicinity of the large-diameter side end portion in the bellows portion are impaired. Therefore, to obtain a desired bend angle, necessary returning times of the large-diameter portion (referred to also as the mountain portion) and the small-diameter portion have to be increased to increase the size of the boot in some case.
To solve the problem, the present inventors have used the small-diameter portion just in the vicinity of the large-diameter side end portion of the bellows portion as an original function of the small-diameter portion of the bellows portion. That is, to obtain a large locking angle (obtain the flexing properties of the small-diameter portion just in the vicinity of the large-diameter side end portion of the bellows portion) while keeping the boot to be compact, the secondary molding has been performed in order to position the end portion of the thick portion in the vicinity of the bellows portion in the vicinity of an outer-diameter direction from the small-diameter portion inner surface just in the vicinity of the large-diameter side end portion of the bellows portion.
Accordingly, the flexing properties of the small-diameter portion just in the vicinity of the large-diameter side end portion of the bellows portion can be obtained. While the boot is kept to be compact, the large rocking angle can be obtained.
However, when a contact position of the end portion of the thick portion in the vicinity of the bellows portion is disposed in the vicinity of the outer-diameter side from the inner surface of the small-diameter portion just in the vicinity of the large-diameter side end portion of the bellows portion, an undercut portion is formed in the thick portion.
In this case, the secondary molding ends, and the boot in which the secondary molded member constituted of a portion having different thicknesses is molded integrally with the large-diameter side end portion inner surface of the primary molded member is pulled out of the core mold. However, since the undercut portion is easily caught by the core mold, the boot is pulled out by so-called forcible extraction. The forcible extraction sometimes deforms or damages the boot as a product. Therefore, the present inventors have considered that the extraction is to be avoided in order to provide a stable product, and have developed the present invention.